Fender Squier 15 amp: mods?

[Plexi]

About 10 years ago, a friend of mine left this amp in my house.

Good amount of volume, but very temperamental: it start to distort in any setting.

The EQ is very extreme: more in the highs.

Is there any way/recomendation to improve this amp?

Maybe relegate the distortion; and make it more compressed, clean and full of bass, to push with distortion pedals.

Schematic:
https://i.imgur.com/U3Lia47.gif

[stringsthings]

I'm guessing that if you disconnect D1 and D2, that might lessen the distortion.
You could probably put the preamp circuit ( with or without the tone circuit ) on a breadboard
and experiment with D1,D2.  They appear to be hard-clipping diodes.

[teemuk]

The clipping happens in two stages: First, symmetrically - and moderately soft - in the shunt diodes, then asymmetrically - and "hard" - in the preceding discrete gain stage.

So earasing diodes won't do it, that will in fact turn the clipping even harder and abrupt.

My suggestion is either to erase the diodes or connect more of them in series (to increase the clipping threshold). AND then change 470R emitter resistance of the gain stage to 4.7K so the stage will operate approximately at unity gain and will not introduce clipping distortion at low signal levels. THEN erase the 22K series resistor preceding master volume control because, as a a side effect to modifications, the preamp will be outputting considerably lower signal magnitude.

Treble and bass response? That calls for modifying 1st stage (like usually, there's "voicing" introduced in the gain control) and the tonestack. I would experiment with those tone control filter component values until you find something that you're pleased with.

Also, it's a ~15W amp. You will not get clean sound out of it at band volumes no matter what. It will start to break up when dialled loud enough.

But personally, I would just hunt for a new amp. These cheapies cost what? 20$ ? Big deal. Also, I regard these cheapies as pretty poor modification platforms to begin with. When modifications actually turn to complete "rebuild" I no longer see the point. IMO, there's more sense to buy a GOOD amp from starters, then mod it to be even better, than in trying to "polish a turd" and basically building and designing a completely new amplifier from scratch. (Rant over).

[Plexi]

Thanks guys..!

About the diodes: one year ago, the volume pot get loose and broke. So, when I opened it, I've replaced it and then lifted the diodes. I don't remember exactly where, but I replaced two resistors too, higher values.
And the response was worst..

I'll try more clippling there, and reduce the Q1 gain with the 470R that you mentioned, teemuk.

And of course, it is a "gift"..something cheaper than that? 

If I can get a bit more "clean" response from it at normal volume, I'll be happy.

Which is the main function of R8 and R7? Volume and Gain pots... I really don't understand how they're working...

[anotherjim]

Vol & Gain looks strange until you notice the ground on the pot wipers (although it's still counter-intuitive!). Gain attenuates negative feedback -  progressively diverting the output signal away from the inverting input. Volume loads the output from the 1k R9 down to ground & at the same time reducing gain by increasing the Rin value via R5.

Not sure, but the tone controls look like a TBM tone stack turned around to work in the op-amp feedback loop. That would mean you don't get the level drop of a conventional amp tonestack.

Remedies I might try include replacing the clipping diodes D1 & 2 with LED's. I'd try green ones. Also reducing the output of the tone op-amp by reducing the fixed feedback resistor R15 below 1M.

[Gus]

Does it have a 8" speaker?
If so try the amp section with a nice 12" speaker(s) in a nice cab.

[Plexi]

Ok... hands on it.





Everything is according the schematic.
You can notice there the lifted diodes..

[Plexi]

Vol & Gain looks strange until you notice the ground on the pot wipers (although it's still counter-intuitive!). Gain attenuates negative feedback -  progressively diverting the output signal away from the inverting input. Volume loads the output from the 1k R9 down to ground & at the same time reducing gain by increasing the Rin value via R5.

Not sure, but the tone controls look like a TBM tone stack turned around to work in the op-amp feedback loop. That would mean you don't get the level drop of a conventional amp tonestack.

Remedies I might try include replacing the clipping diodes D1 & 2 with LED's. I'd try green ones. Also reducing the output of the tone op-amp by reducing the fixed feedback resistor R15 below 1M.

Thanks Jim for the explanation and advices!

[Plexi]

Updates at the moment:
- Reduced R15 to 500k
- Increase R1 to 430K
- Increased R20 to 4k7
- Removed R8, and place a 22k resistor from C4 to ground.

Leave lifted both diodes.

At the moment, it sounds cleaner and less strident. With gain at the max, start to broke a little.


Advices to make it more darker, in the EQ section?

[Plexi]

Update 2:
- C1 and C15 increaded from 100nF to 220nF.
- R6 increased to 10k

Now it sounds GREAT.
Not crispi highs, great lows: take distortion pedals like a charm.

[Plexi]

Little beast amp 

[Extremesecrecy]

Hi!
These are the steps I see you did.
- Reduced R15 to 500k
- Increase R1 to 430K
- Increased R20 to 4k7
- Removed R8, and place a 22k resistor from C4 to ground.
- C1 and C15 increased from 100nF to 220nF.
- R6 increased to 10k

Do you have a picture of this one?
- Removed R8, and place a 22k resistor from C4 to ground.

[Plexi]

Hi!
These are the steps I see you did.
- Reduced R15 to 500k
- Increase R1 to 430K
- Increased R20 to 4k7
- Removed R8, and place a 22k resistor from C4 to ground.
- C1 and C15 increased from 100nF to 220nF.
- R6 increased to 10k

Do you have a picture of this one?
- Removed R8, and place a 22k resistor from C4 to ground.

Hi! I don't.
Just as it is some kind of voltage divider to ground, just place 22k resistor where pot whip 3 is, to ground or middle pot whip. Whip 1 leave with no conection, since at 0, it take all the signal to ground. At 50% way of the pot, sent or bypass Cap C4 to ground, limited by R5. If there's no resistance, just Cap to ground, distortion will be massive and ugly. That way, it tames a lot, and avoid pot cracking or failure, how as I've remember it was doing.